Various methods of detecting particles in air are known. One method of detecting the presence of particulate matter in air involves projecting a beam across a monitored area and measuring the attenuation of the beam. Such detectors are commonly known as ‘obscuration detectors’, or simply ‘beam detectors’.
An exemplary, conventional beam detector is shown in FIG. 1. The detector 100 includes a light emitter and detector 102 and a reflector 104 placed either side of a monitored area 106. Incident light 108 from the light emitter and detector 102 are projected toward the reflector 104. The reflector 104 reflects the incident light 108 as reflected light 110. Reflected light 110 is reflected back toward the light source and detector 102. If particulate matter enters the monitored area 106, it will attenuate the incident light 108 and reflected light 110 and cause the amount of light received at the light source and detector 102 to diminish. An alternative beam detector omits the reflector and directly illuminates the detector with the light source across the monitored area 106. Other geometries are also possible.
Whilst the mechanism of smoke detection used by beam detectors is sound, beam detectors commonly suffer from a number of problems.
Firstly, beam detectors may suffer a type I (false positive) error where foreign objects or other particulate matter, such as dust, enters the monitored area and obscure the beam. Beam detectors are generally unable to distinguish between the obscuration caused by particles of interest e.g. smoke, and absorption which results from the presence of foreign body of no interest e.g. a bug flying into the beam.
Secondly, beam detectors may require careful alignment at the time of installation. Such alignment aims to ensure that in normal conditions, free from smoke, light enters the sensor so as to capture the majority of the transmitted beam, and to in turn maximise sensitivity to an obscuration. This calibration may be slow and therefore costly to perform. Moreover, it may need to be repeated as the physical environment that the detector occupies changes, for example because of small movements in the structure to which a beam detector is attached. In some cases, if the intensity of incident light on the detector diminishes quickly this misalignment may also cause a false alarm.
One way of compensating for the second problem is to introduce a photodetector having a high sensitivity over a wide range of incident angles. This reduces the effect that poor alignment between the beam and photodetector would otherwise have. However, this solution comes at the cost of increased sensitivity to unwanted background light, which in turn complicates the detection process and increases the likelihood of failing to detect the presence of particles of interest.
Supplying power to the transmitters within a particle detection system can be costly. There are practical/commercial limits on the amount of power that can be supplied. The limited supply of power limits the optical power output of the transmitter, which in turn limits the signal to noise ratio of the measured signal. If the signal to noise ratio of the system degrades too far, the system may experience frequent or continual false alarms.
In some systems, the signal to noise ratio can be enhanced by employing long integration or averaging times at the receiver. However system response times, which are usually between 10 and 60 seconds, must be increased to higher levels if long integration times are used. This is undesirable.